I'm not a technician but i like to understand things how they work en how I can improve them. I'm very pleased with my meridian 605 monoblocks. Though they are working very fine i would like to updrage them. My idea is following:

Question: the opamp is an OP44 and I learnerd it's an important part for the sound. Is there an upgrade that keeps the meridian sound ( afraid of losing it) and delivers even more transparency en smoothnes in high frequencies.

1. Replace the two 4700uF Cerafines with four 10,000uF. I suggest something Nichicon Great Supply 10,000 @ 80V. You can't easily fit the 'Super' as they are 63mm wide, that though is what I'm using. You may have the use the lower grade 50mm diameter 'Gold'.
2. Yes, it's much better. That VDH stuff sounds, err, just get rid of it.
3. I can't comment. But changing just the 9 inches of VDH from the RCA jack to the input pin to solid silver was easily audibly better.

I actually fitted a terminal post to the earth point on the main capacitor bank and the sound was quite a bit better with the wires plugged into there than the terminal post on the back and running through all that internal wire. I run them with the lids off to do this. And anyway the caps are not in the case due to their size.

There are other things you can do.

Larger mains fuse; the risk is yours. I have a home made 15 amp in there.

A very thick mains leads helped the whole sound enormously.

I replaced the wire with 6mm2 from the mains switch to the transformer, via the relays. Really, that single pole mains switch and single pole relay is degrading the sound quality I expect. I just tried a 16 pole relay on another amplifier and even that was a downgrade over a captive lead soldered direct onto the transformer wires. My plan with the 605 was to add to each mono block two bifurcated 4 poles relays (so 16 contacts) with a soft start circuit so as to bypass the switch and relay, but still retain the switch function and that relays cut off protection. Just not feed the Tx through them.

I added a 0.1uF X-cap across from the IEC inlet to the Tx. Then another two. Really lifted the treble, but the amps don't full switch off; the LED stays on a little bit they are powered up enough to make hum emit from the speakers. My fix for this was to leave them there, keep the treble, and plug / unplug the wall sockets to power the amps up / down.

I changed the 5 volts Zeners to 13 Volt ones, 1.3 Watt; they don't fit the original small holes, had to shave the Zeners leads a bit.

I fitted Vishay bulk foil for the 47K feedback resistor. Well worth doing.

Changed the output coil to the same number of turns of thicker enamelled copper wire. Gave quite a bit stronger bass.

Changed the output resistor to a Caddock MP130. More treble but a bit shrill perhaps.

Also fitted a 22uF polystyrene across each MOSFET. Normally Gate to Drain, ie, the outer two pins. But if I recall correctly, the 605 has the MOSFETS the other was around so power is fed to the middle pins, ie, the Source. I put heatshrink around the body of each cap as they are under the PCB and it's weight can fall on them when fitting it back into the case. This mod is there is add stability but it did change the sound.

I added some extra caps, BG's, Nichicons, and Oscons, pre and post regulators. Don't know if this made any difference.

I've not yet tried alternative Op-Amps. Just in the process of doing that now.